The increasing demand for smaller, more compact electronic devices has led to extensive research and development in the field of microelectronic circuitry. As a result of this effort, major advances have been made in the field in the past decades. However, it is becoming increasingly apparent that the field of miniaturization has reached, or at least is rapidly approaching the point at which further progress will be increasingly difficult to achieve unless new and innovative approaches are resorted to.
In this connection, the scientific community has for some time been aware that certain biological systems make use of organic molecules to store and transfer electrical charges. Bilipid membranes consisting of phospholipid bilayers having globular proteins located therebetween, for example, perform such functions in nature. In view of the existence of these and similar systems, the attention of researchers is increasingly being focused on the use of organic films, including both mono-molecular, and multi-layer films to perform electronic functions.
In particular, interest has been directed to the use of Langmuir-Blodgett films formed from a variety of organic molecules to produce highly oriented, uniform films from about 10-5,000 nanometers thick. The formation of Langmuir-Blodgett films involves the deposition of amphiphilic molecules onto a water surface where they form a mono-molecular film. The molecules dispose themselves across the surface of the water in an oriented configuration in which their hydrophilic end is positioned adjacent to the surface of the water, while their hydrophobic end is directed away therefrom. After deposition, commonly from a relatively volatile solvent, the molecules are forced together by means of a movable hydrophobic barrier which compresses the molecules into a continuous film, one molecule thick. Substrates can thereafter be passed through the film in a direction perpendicular thereto, resulting in the coating of the substrates with a monomolecular film of the material. Molecules in the film are disposed adjacent and parallel to each other at an angle to the surface of the substrate, in some cases approaching 90.degree. C. By repeatedly passing the substrates through the film, or sequentially through different films, multiple-layer film structures can be prepared, either consisting of identical molecular layers, or alternating, different molecular layers, as the case may be.
The Langmuir-Blodgett techniques thus permit "engineered" films to be formed from specific materials, in whatever thickness is required. The thickness can be controlled, for example, by the number of layers of molecules deposited on the substrate, as well as by the length of the molecules employed to form the films. In addition, when molecules exhibiting polar characteristics are used to form such films, the dipole moment of the molecules can provide a difference in electric potential across their length sufficient to allow electronic functioning to be realized, including pyroelectric, piezoelectric and non-linear optical effects.
In order for films to demonstrate such functioning, it is necessary that the molecules comprising them have a polar axis resulting from their spontaneous polarization. In instances where the spontaneous polarization of the molecules varies with the temperature or pressure to which the molecules are exposed, the neutralization of the polarization charge at the surface of films made from the molecules by ions present in the surrounding atmosphere, or from other causes, is too slow to prevent detection of a charge in the amount of the surface charge. It is this change in spontaneous polymerization with temperature, or pressure, termed the pyroelectric coefficient in the case of temperature, and representing the change in spontaneous polarization per degree of temperature change, on which the successful use of such films in certain detection devices is predicted. The measurable charge thus produced is substantially proportional to the product of such coefficient and the temperature change.
Materials having pryoelectric characteristics have been known for a considerable period of time. Ferroelectrics, for example, are among the more conventional pyroelectric materials, and while ferroelectrics show relatively high pyroelectric coefficients, they must be operated at relatively high heat levels, e.g., near their Curie temperature, in order to obtain the higher changes in spontaneous polarization values with temperature of which they are capable.
In the organic area, films formed from vinylidene fluoride have in the past been prepared which also display pyroelectric effects. Such films are relatively thick, however, and require substantial energy inputs to raise their sensible temperatures. Thus, they tend to suffer from a lack of thermal sensitivity. In addition, such films must initially be "polled", i.e., exposed to externally applied magnetic flux to activate them.
Considerable work has also been done using Langmuir-Blodgett techniques to develop pyroelectric materials. Various saturated fatty acids have, for instance, been combined with stearylamine, and the latter compound has also been combined with unsaturated omega-tricosenoic acid, the films comprising alternating layers of such materials. The aforementioned films display pyroelectric coefficients of varying values, at least some of which are of sufficient magnitude to be useful in pyroelectric measurements. However, as in the case of the vinylfluoride materials, such films have in the past involved the lamination of a relatively large number of molecular layers, for example, 500 layers. Such laminations, therefore, not only entail excessive processing for their fabrication, but in addition, the thickness of the resulting structures requires a greater heat input to produce a given rise in temperature. Consequently, the laminates are comparatively insensitive relative to thinner films, the latter being much more desirable.
Furthermore in instances where the fabrication of thinner laminations is attempted, the resulting films tend to be fragile and susceptible to damage when physically stressed, imposing limitations on the applications in which they can be used.